Composite thin films of CdSe nanocrystals and a surface passivating/electron transporting block copolymer: Correlations between film microstructure by transmission electron microscopy and electroluminescence

Abstract

We present a microscopic characterization of composite thin films made of CdSe semiconductor nanocrystals (quantum dots) dispersed in a polynorbornene-based block copolymer with surface-passivating and electron transport functionalities. We used two types of nanocrystals, CdSe with pure organic capping groups (“bare”) and CdSe–ZnS core-shell nanocrystals, also capped on the outer surface with organic groups. The composite thin films are incorporated in heterostructure light emitting devices where they serve as both the electron transport layer and the emissive layer. A thin layer of self-assembled poly (phenylene vinylene) (PPV) is used as the hole transport layer. We used transmission electron microscopy (TEM) to examine the film structure in plan view and in cross section. The TEM analysis uncovered distinct and complex differences between the microstructures of thin films containing bare and ZnS overcoated nanocrystals. We found a strong correlation between the composite film microstructure and the corresponding electroluminescence (EL) spectra as well as the device performance. For example, the presence of phase separation at low concentration and nanocrystal migration to the surface in films with ZnS overcoated particles, result in a large PPV contribution to the EL spectra. At higher concentrations, particles form connecting paths between the interfaces, resulting in spectra dominated by nanocrystal emission, but with a higher efficiency for bare particles.